Ronaldinho strikes thrice vs Siena
The title race is now very much on given Inter only drew 2-2 at Bari on Saturday, Milan have a game in hand and the top two meet in the city derby next weekend.
Juventus are out of the running though after a 1-0 defeat at Chievo, their fifth loss in seven games in all competitions.
Gennaro Sardo netted for the dominant hosts in the first half but under-pressure Juve coach Ciro Ferrara can blame his terrible injury problems, which meant Michele Paolucci started upfront having been recalled from Siena only on Saturday.
Claudio Ranieri's AS Roma have leapfrogged former club Juve into third, 11 behind Inter, after on-loan striker Luca Toni grabbed his first two goals for the club from close range in a 3-0 home win over Genoa.
Roma travel to Juve next Saturday when a section of Turin's Stadio Olimpico will be closed because home fans continue to chant racist songs about Inter striker Mario Balotelli.
Napoli are fourth, a point above fifth-placed Juve, following a lucky 0-0 home draw with Palermo in the "Derby of the South" between two of Italy's best-supported teams.
Palermo's Fabrizio Miccoli had a scuffed penalty saved when he became the latest Serie A player to have a laser shone in his face by fans, while team mate Simplicio missed a near open goal.
Milan had a poor start to the season under new coach Leonardo but their remarkable renaissance has largely been thanks to Ronaldinho and Marco Borriello, who like Toni could now make a late bid to gatecrash Italy's World Cup squad.
Striker Borriello won a 12th-minute penalty when he was brought down by keeper Gianluca Curci, who was given a straight red card, and Ronaldinho confidently slotted in the spotkick.
Borriello soon volleyed in Andrea Pirlo's cross with his wrong foot to prompt a standing ovation at the San Siro.
Ronaldinho, reborn after recapturing some of the form which made him 2004 and 2005 world player of the year, then nodded in David Beckham's second-half corner and fired in a superb shot which rekindled memories of his glory days at Barcelona.
Siena, unlucky to lose 4-3 against Inter last weekend, slipped four adrift at the bottom after relegation rivals Catania drew 1-1 at Sampdoria and Atalanta stunned Lazio 3-0 at home in Bortolo Mutti's first game in charge.
Bologna also boosted their survival hopes with a surprise 2-1 win at faltering Fiorentina while fellow strugglers Udinese managed a 0-0 draw at Parma in a bad-tempered game.